The present invention relates generally to Light Emitting Diode “LED” lighting systems and more particularly LED lighting systems suitably adapted for airfield lighting (e.g. runway, taxiway and obstruction lights)
Airport edge lighting has been in existence for many years utilizing incandescent lighting technology. Conventional designs that utilize incandescent lights have higher power requirements, lower efficiency, and low lamp life which needs frequent, costly relamping by maintenance professionals.
Some airfield-lighting manufacturers are using more efficient devices such as Light Emitting Diodes (LEDs) where the LEDs are arranged in multiple rings shining outward. Optics are employed to concentrate the light in the vertical and horizontal directions to meet Federal Aviation Administration (FAA) specifications.
LEDs are current driven devices. A regulated DC current flows through each LED when the LED is conducting. There are two primary concerns with a pure DC power source. First, a field insulation resistance fault may degrade faster (corona or arc welder effect) and second, dimming.
Dimming is usually accomplished by reducing DC current, however LEDs are not reliable when operating at lower current levels. For example, LEDs available from Philips Lumileds Lighting Company, 370 West Trimble Road, San Jose, Calif., 95131 USA, Phone: (408) 964-2900, are on a die that contains many individual LED structures. If enough current is not provided, the current is not evenly distributed across the die, causing uneven illumination. Operation below 100 mA becomes extremely sporadic, and the LEDs may fail to light at all. Also, luminous flux output between devices is extremely uneven.